Testosterone (T) and dihydrotestosterone (DHT) are the major androgens in the human prostate and are required for normal prostate development and homeostasis. In men, aging is associated with a gradual decline in serum T levels. Despite declining serum T concentrations, the incidence of prostate diseases increases markedly with age. Androgen replacement therapy in older men with age-associated androgen deficiency results in beneficial anabolic effects on muscle and bone, and can improve sexual function and strength, yet significant concern exists regarding the possibility of potentiating prostate disease. The goal of therapeutic androgen therapy is thus to maximize anabolic effects while minimizing effects on the prostate. Previous studies have demonstrated that the anabolic benefits of T exhibit linear dose-response effects but suggest that the prostate does not exhibit similar dose-responses to increases in serum androgens. We hypothesize that the dose-response effects of exogenous androgens within the prostate are not linear because the prostate maintains a unique androgenic milieu in the face of changes in serum androgens. In Aim 1 of the proposed studies, we will determine whether increasing concentrations of serum T alter the intraprostatic androgen environment and have downstream cellular consequences within the gland. Within the prostate, the enzyme 51-reductase (51R) is highly expressed resulting in a high intraprostatic concentration of DHT, which has been implicated in the development of prostate disease. In Aim 2 we will determine the hormonal and molecular effects of low and high doses of T in the presence of a 51R inhibitor within the prostate to better characterize this prostate sparing androgen replacement strategy. In Aim 3 we will investigate whether the healthy human prostate has the capacity to synthesize DHT and T in situ from adrenal precursors, a mechanism which might contribute to stabilizing intraprostatic androgen concentrations when serum levels fluctuate. We will perform our interventions in humans where we have experience with prostate tissue analyses including quantification of hormones and cell-specific gene expression analysis as a marker of androgen action. These studies will provide an understanding of the hormone-related changes in the prostate microenvironment and will help inform future studies of androgen replacement therapies in older men.